1. Field of the Invention
The present invention relates to fabrication of a glass substrate. More particularly, the invention relates to an improvement of the surface properties of a glass substrate for a magnetic disk to form a hard disk which is used as a large scale recording medium in computers.
2. Description of the Background
Conventionally, although a substrate which is prepared from an aluminum alloy and the surface of which is plated with nickel, including a phosphor, has widely been used as a substrate for a magnetic disk, the demand for a glass substrate in such a role has been increasing in recent years, because of the necessities of small size hard disk drives, high recording densities of the disks, as well as low flying height of the magnetic head, promotion of impact strength and the like.
A glass substrate for a magnetic disk is required to have the properties of withstanding the centrifugal force caused by rotation of a disk drive and impact force which results upon collision of the substrate with a magnetic head and the like. Further, a glass substrate is required to have a preferred orientation of magnetic material on it so that the substrate has a sufficient S/N ratio.
In order to improve the above mentioned mechanical properties, the following two glasses are used which are a) chemically strengthened glass prepared by replacing Na of one of the glass ingredients with K and b) crystallized glass in which the fine crystals precipitate into the glass matrix and give rise to strain for strengthening the matrix.
As chemically strengthened glass is treated in an alkaline ion exchange process, the glass contains a significant amount of alkaline material in its surface. Since a gel material is formed on the glass surface during polishing, the hardness of the surface becomes weak, and the water present in glass is released during sputtering of a magnetic layer in a vacuum. This causes the magnetic layer to have a low orientation and a poor SIN ratio. Moreover, when the gel reacts with water and CO2 gas, projections are formed on the surface of the glass disks. Therefore, the to conventional approach to cleaning glass substrates immediately before a medium fabrication step, is to clean the glass substrate with sulfuric acid and phosphoric acid solution immediately after chemical treatment of the glass to remove an alkaline component on the surface (Japanese Unexamined Patent Publication No. JP-A-9-22525). In this process alkaline components are removed, but since the oxidation activity is not sufficient to repair the structure of glasses, the surface hardness is low with the result that the surface corrodes. That is, as the glass structure of the Sixe2x80x94O combination is destroyed by the alkaline component present in the polishing solution, the surface hardness is still low and the magnetic layer which is formed after polishing has poor crystalline orientation. Besides, the conventional method of cleaning glass to remove alkali therefrom after polishing is not sufficient to prevent corrosion. Although it has been proposed to provide the glass surface with a silicon oxide coating by the alkoxide method after removing alkali, a heat treatment at more than 600xc2x0 C. is necessary to form perfect the SiO2 coating. This temperature is much higher than the strengthening point of 400 to 480xc2x0 C., and it is not possible to form complete the SiO2 coating.
Generally speaking, a glass structure of the Sixe2x80x94O combination, even in crystallized glass which does not contain alkali, is destroyed in mechanochemical polishing. Therefore, the surface of crystallized glass turns to a gel, as well as ion-strengthened glass, the orientation of magnetic layer formed on the gelled surface becomes random, and the resulting soft glass surface exhibits deteriorated corrosion resistance, as well as an anti-head impact property. A need, therefore, continues to exist for an improved method of strengthening the surface of glass while achieving improved corrosion resistance and anti-head impact properties of the surface.
Accordingly, one object of the present invention is to overcome the above-described problems, which include achieving corrosion resistance and anti-head impact property, of strengthened glass and crystallized glass which are employed for the fabrication of magnetic recording substrates and electric component mounting boards by conventional technology in order to achieve improvement of the properties of the glass surface and the orientation of magnetic layer which is formed on the glass surface.
Briefly, this object and other objects of the present invention as hereinafter will become more readily apparent can be attained by a method of cleaning a glass substrate, comprising contacting a preliminarily polished glass disk with ionized water of which the ORP potential is positive for a predetermined period of time in the last rinsing step in order to repair the structure of the glass so as to achieve higher surface hardness and improved corrosion resistance.